1. Field of the Invention
The present invention relates to a pipeline system for the distribution of a flowing medium, comprising a main line which branches at a branching point into a plurality of branch lines, in each of the branch lines a variable restrictor, by means of which the mass flow in each of the branch lines can be adjusted, and, belonging to each restrictor, a first pressure measuring device, by means of which the pressure drop of the flowing medium at the respective restrictor is measured.
The invention relates, furthermore, to a method for operating such a pipeline system.
2. Discussion of Background
In power station technology or even other areas of use, there is often the task of supplying a multiplicity of consumers with a mass flow of a compressible or incompressible medium (for example, cooling water, steam, oil or the like). The supply system used for this purpose consists typically of a network of pipelines which is distinguished by branching points (junction points), at which a main line (a main stream of the medium) branches into two or more branch lines, (branch streams) which lead to the individual consumers or groups of consumers. In many instances, it is necessary, in this case, for the mass flow to be controlled in each individual branch line according to the requirements of the consumer or consumers. For this purpose, for example, a control valve may be arranged in the branch line, the lift of said control valve being adjusted in such a way that the desired mass flow flows through the valve.
A simple way of controlling the mass flow of the medium by means of a control valve is to calculate the valve lift which is required in order to produce the predetermined mass flow. The calculation of the valve lift is typically based on the pressure loss (pressure drop) measured at the control valve, on the characteristic of the valve and on the properties of the medium. In the simplest instance, a pipeline system, as represented in FIG. 1, is then obtained (for example, for the fuel supply system of an industrial gas turbine). In the pipeline system 10 of FIG. 1, a main line 11 branches at a branching point 12 into (for example) three branch lines 13,14 and 15. Provided in each of the branch lines 13,14,15 is a valve V1 or V2 or V3, by means of which the mass flow through the respective branch line can be adjusted (controlled). Arranged parallel to the valve V1,V2,V3 in each case is a pressure measuring device PM1 or PM2 or PM3 which measures the pressure drop at the valve.
If the valve lift of the valves V1, . . . ,V3 is designed by h, then h is a function of the valve characteristic K.sub.V, namely EQU h=h(K.sub.V). (1)
For a compressible medium (for example, the fuel gas for the gas turbine), the quantity K.sub.V for sub-critical flow conditions is obtained as EQU K.sub.V =.alpha.(dm/dt)[T.sub.M /(p.sub.M -.DELTA.p)].sup.1/2 [1/.DELTA.p].sup.1/2, (2)
with the constant .alpha., the mass flow dm/dt, the pressure P.sub.M and the temperature T.sub.M at the branching point 12 and in the main line 11 respectively, and the pressure drop .DELTA.p at the valve. For a predetermined mass flow dm/dt, the quantity K.sub.V can be determined on the basis of the measured quantities T.sub.M, p.sub.M and .DELTA.p according to equation (2). The valve lift can be calculated from this from the predetermined valve characteristic K.sub.V (h). A comparable determination can also be carried out for the incompressible media.
The most important quantity for calculating the valve lift is the pressure drop measured at the valves V1, . . . ,V3. If this measurement becomes defective, this leads to an unacceptable failure of the supply system (and, in the case of a gas turbine, to an emergency shutdown) or even (for example, in the case of a cooling water system) to a safety risk. It is therefore desirable, in many instances, to make the measurement of the pressure drop at the valves V, . . . ,V3 redundant, so that a fault in an individual measurement of the pressure drop .DELTA.p does not affect or impair the continuous reliable operation of the plant (availability requirement AR).
The purpose of a redundancy concept is twofold: (1) the occurrence of a measuring fault is to be recognized and the faulty measuring device and faulty measuring channel are to be identified. (2) The (non) useable measured values are to be replaced by measured values determined redundantly.
Two fundamental types of fault are to be taken into account here:
Notified Faults (Notified Failure NF):
This type of fault embraces all the faults which are notified to the control system by the transmitter or another I/O device by means of a bad data quality (BDQ) signal. The control system knows from the BDQ signal which Ap signal is faulty. This occurs typically when a measuring line is interrupted or a fault occurs in a component in a measuring chain.
Drift in Measurement:
This type of fault describes the creeping deterioration of the measurement signal, so that the transmitted information is no longer a valid measurement of the pressure drop. It cannot be detected and is therefore also not notified to the control system. Other ways of handling this type of fault must therefore be adopted.
The redundant measurement of the pressure drop may be carried out with double redundancy according to FIG. 2. In the case of double redundancy, in addition to the pressure measuring device PM1, . . . ,PM3 already present a second pressure measuring device PM4, . . . ,PM6 is in each case arranged in parallel for each valve. If one of the two pressure measurements (per valve) is faulty, there can be a changeover to the other pressure measurement. However, this is possible only for notified faults, in which the faulty measurement can be detected by means of the BDQ signal. By contrast, a drift in the measurement cannot be overcome by means of double redundancy, since, with only two independent measurements per valve, it is not possible to decide which of the two measurements is disrupted (or is drifting).
To overcome this problem, the redundant measurement of the pressure drop may be carried out with triple redundancy according to FIG. 3. In the case of triple redundancy, in addition to the pressure measuring device PM1, . . . ,PM3 already present a second pressure measuring device PM4, . . . ,PM6 and a third pressure measuring device PM7, . . . ,PM9 are in each case arranged in parallel for each valve. The 2 of 3 selection principle is employed to determine the faulty measurement in the case of drift. In the 2 of 3 selection principle, it is assumed that, if 2 of 3 measuring channels give the same measured values, these measuring channels are working faultlessly, whilst the third measuring channel is faulty.
Both in the case of double redundancy illustrated in FIG. 2 and, in particular, in the case of triple redundancy illustrated in FIG. 3, there is the disadvantage that a very large number of independent pressure measuring devices PM1, . . . ,PM6 or PM1, . . . ,PM9 must be used, thus involving considerable outlay, particularly in the case of triple redundancy with three pressure measuring devices per branch line.